The invention relates to a solid electrolytic capacitor having manganese dioxide as a semiconductor compound and an anode body formed of a film-forming metal which is provided with a dielectric oxide film by means of anodic oxidation.
Manganese dioxide is obtained by moistening the oxidized anode body or by impregnating it with a solution of a maganese salt which after heating is converted into manganese dioxide by pyrolysis.
The shape of the cathode depends on the specific embodiment of the capacitor. In a solid tantalum capacitor having a porous sintered anode or a capacitor having an anode which is punched from aluminum sheet, etched and folded, the cathode comprises a multiple layer of graphite, silver and tin, which is deposited on the formed, impregnated and pyrolysed anode body. In a solid aluminum foil capacitor, the cathode comprises an etched aluminum foil which is wound up together with at least two separator foils.
The capacitor is incorporated in a housing of an electrically insulating material, comprising, for example, silicone resin, epoxy resin, polystyrene or polyester.
Impedance Z at 100 kHz and the loss factor (tan .delta.) of these capacitors generally increase during operation at elevated temperatures and for an extended period of time while the capacitance decreases.
In a life test which is carried out at 125.degree. C., the requirement is for the increase of Z and tan .delta. to be less than 20% after 2000 hours.
It is assumed that the increase of the impedance Z and of tan .delta. is caused by an dissociation of MnO.sub.2 at a elevated temperature, thereby releasing oxygen. Subsequently, the this oxygen reacts with the material of the housing which is gradually decomposed. Thus, the dissociation of the manganese dioxide becomes irreversible, the lower manganese oxides being responsible for a decrease in conduction.
GB-PS No. 885,965 which relates to a solid electrolytic capacitor as referred to in the opening paragraph, describes the above-mentioned properties of such a capacitor during use at high temperatures. A decrease of capacitance and increase of leakage current can be reduced in accordance with this publication by introducing oxygen or an oxygen-furnishing substance in the hermetically sealed housing. Silver peroxide, barium peroxide or perchlorates are listed as the sources for the oxygen-furnishing compound.
However, from a view-point of industrial manufacture, there are some inconveniences attached to the introduction of oxygen or an oxygen-furnishing substance. Moreover, the introduction of substances which are alien to the capacitor and which are rather agressive should be avoided in order not to reduce the electrical properties of the capacitor.